510 The Tides in Relation to Geophysical and Cosmic Problems 



p. 131) has calculated an ideal, simplified case, which comes as close as poss- 

 ible to conditions existing in nature. The simplifications consist, in the first 

 place, in the fact that the earth is assumed to be level, which assumption can 

 be made without hesitation, inasmuch as the tidal load affects only the upper 

 layers close to the surface; secondly, the ocean is assumed to consist of an inde- 

 finite number of wide canals separated from each other by wide strips of land 

 of the same width. It is further assumed that in these seas the water oscillates 

 in the form of simple oscillations (seiches) around a central line, so that when 

 there is high water on one side there is low water on the other side and vice 

 versa. The problem is to find out how the tides change the configuration of 

 the ocean bottom and the strips of land. The result of the computation is 



OCEAN CONTINENT OCEAN CONTINENT 



Fig. 209. Deformation of land and ocean bottom by the tidal load (Darwin). 



shown in Fig. 209, in which the resulting bottom slope is exaggerated for the 

 sake of clearness. Through the pressure of the water the surface of the land 

 and of the sea, which were level prior to the tidal disturbance, takes the form 

 of the curved line when there is low water to the left of the strip of land and 

 high water to its right. It we interchange N.W. with L.W.,the figure is reversed 

 like a mirror-image. One notices that both the solid earth and the ocean bottom 

 oscillate around a central position, which shows that the strip of land appears 

 to be nearly level, the ocean bottom somewhat curved. The sharp bend in the 

 coast line is due to the assumption of a discontinuous form of the solid earth 

 along the shore line ; it vanishes if one assumes that the depth of the ocean in 

 front of the coast does not increase suddenly, but gradually. If one assumes 

 that the range of the tide on the coast is 160 cm, the width of the oceans and 

 continents 6280 km each (which corresponds to the average width of the 

 Atlantic Ocean) and that the rigidity of the rocks of the earth is twice that 

 of the most flexible glass and a quarter that of the hardest glass, then the 

 slopes of the land caused by the tidal load at high water are given in Table 86. 

 At low water, the inclinations occur in the opposite direction, so that the 

 variation in inclination during an entire tidal period is double the amount 

 listed in the table. To make the order of magnitude of these inclinations 

 clearer, we will state that two-hundredths of arc second corresponds to an 

 inclination of 1 cm on 103 km; a pendulum at the coast would participate 

 to the fullest extent in this change in inclination of its base. 



